As approaches used to solve systems of linear equations using iterative methods (or iterative operations), the Gauss-Seidel method and the Gauss-Jacobi method are known in the related art. In the analysis of a water pipe network by using a circuit simulator, there is known a technique in which a virtual state equation is solved by using the Gauss-Seidel method (for example, see Japanese Laid-open Patent Publication No. 10-320444).